Circulation
Sipunclans do not have a vascular blood system. Fluid transport and gas exchange are instead accomplished by the coelom which contains the respiratory pigment haemerythrin, and the tentacular system. The coelomic interstitial fluid contains five types of coelomic cells: haemocytes, granulocytes, large multinuclear cells, ciliated urns and immature cells. The ciliated urn cells may also be attached to the peritoneum and assist in waste filtering from the interstitial fluid. Nitrogenous waste is excreted through a pair of metanephridia opening close to the anus, except in Phascolion and Onchnesoma which have only a single nephridium. A ciliated funnel, or nephrostome, opens into the coelomic cavity at the anterior end, close to the nephridiopore.
The tentacular system connects the tentacles at the tip of the introvert to a ring canal at their base, from which a contractile vessel that runs along the esophagus and ends blindly posteriorly. Some evidence points towards their involvement in ultrafiltration.
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